In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. This is particularly true for multicolor cinematic projection systems. Competitive digital projection alternatives to conventional cinematic-quality projectors must meet high standards of performance, providing high resolution, wide color gamut, high brightness, and frame-sequential contrast ratios exceeding 1,000:1.
Increasingly, the motion picture industry has moved toward the production and display of 3 dimensional (3D) or perceived stereoscopic content in order to offer consumers an enhanced visual experience in large venues. While entertainment companies such as Disney have offered this content in their theme parks for many years and Imax has created specialty theatres for such content, in both those cases film has been the primary medium for image creation. To create the stereo image, two sets of films and projectors simultaneously project orthogonal polarizations, one for each eye. Audience members wear corresponding orthogonally polarized glasses that block one polarized light image for each eye while transmitting the orthogonal polarized light image.
In the ongoing transition of the motion picture industry to digital imaging, some vendors, such as Imax, have continued to utilize a two-projection system to provide a high quality stereo image. More commonly, however, conventional projectors have been modified to enable 3D projection.
The most promising of these conventional projection solutions for multicolor digital cinema projection employ, as image forming devices, one of two basic types of spatial light modulators (SLMs). The first type of spatial light modulator is the Digital Light Processor (DLP) a digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex.
FIG. 1 shows a simplified block diagram of a projector apparatus 10 that uses DLP spatial light modulators. A light source 12 provides polychromatic unpolarized light into a prism assembly 14, such as a Philips prism, for example. Prism assembly 14 splits the polychromatic light into red, green, and blue component wavelength bands and directs each band to the corresponding spatial light modulator 20r, 20g, or 20b. Prism assembly 14 then recombines the modulated light from each SLM 20r, 20g, and 20b and provides this unpolarized light to a projection lens 30 for projection onto a display screen or other suitable surface.
DLP-based projectors demonstrate the capability to provide the necessary light throughput, contrast ratio, and color gamut for most projection applications from desktop to large cinema. However, there are inherent resolution limitations with existing devices typically providing no more than 2148×1080 pixels. In addition, high component and system costs have limited the suitability of DLP designs for higher-quality digital cinema projection. Moreover, the cost, size, weight, and complexity of the Philips or other suitable combining prisms are significant constraints.
The second type of spatial light modulator used for digital projection is the LCD (Liquid Crystal Device). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. LCDs appear to have advantages as spatial light modulators for high-quality digital cinema projection systems. LCOS (Liquid Crystal On Silicon) devices are thought to be particularly promising for large-scale image projection. However, LCD components have difficulty maintaining the high quality demands of digital cinema, particularly with regard to color, contrast, as the high thermal load of high brightness projection affects the materials polarization qualities.
Conventional methods for forming stereoscopic images from these conventional micro-display (DLP or LCOS) based projectors have been based around two primary techniques. The less common technique, utilized by Dolby Laboratories, for example, is similar to that described in US Patent Application Publication No. 2007/0127121 by Maximus et. al., where color space separation is used to distinguish between the left and right eye content. Filters are utilized in the white light illumination system to momentarily block out portions of each of the primary colors for a portion of the frame time. For example, for the left eye, the lower wavelength spectrum of Red, Blue, and Green (RGB) would be blocked for a period of time. This would be followed by blocking the higher wavelength spectrum of Red, Blue, and Green (RGB) for the other eye. The appropriate color adjusted stereo content that is associated with each eye is presented to each modulator for the eye. The viewer wears a corresponding filter set that similarly transmits only one of the two 3-color (RGB) spectral sets. This system is advantaged over a polarization based projection system in that its images can be projected onto most screens without the requirement of utilizing a custom polarization-maintaining screen. It is disadvantaged, however, in that the filter glasses are expensive and the viewing quality can be reduced by angular shift, head motion, and tilt. Additionally, adjustment of the color space can be difficult and there is significant light loss due to filtering, leading to either a higher required lamp output or reduced image brightness.
The second approach utilizes polarized light. One method, assigned to InFocus Corporation, Wilsonville, Oreg., in U.S. Pat. No. 6,793,341 to Svardal et al., utilizes each of two orthogonal polarization states delivered to two separate spatial light modulators. Polarized light from both modulators is projected simultaneously. The viewer wears polarized glasses with polarization transmission axes for left and right eyes orthogonally oriented with respect to each other. Although this arrangement offers efficient use of light, it can be a very expensive configuration, especially in projector designs where a spatial light modulator is required for each color band. In another approach, a conventional projector is modified to modulate alternate polarization states that are rapidly switched from one to the other. This can be done, for example, where a DLP projector has a polarizer placed in the output path of the light, such as at a position 16 indicated by a dashed line in FIG. 1. The polarizer is required as the DLP is not inherently designed to maintain the polarization of the input light as the window of the device package depolarizes due to stress induced birefringence. An achromatic polarization switcher, similar to the type described in US Patent Application Publication No. 2006/0291053 by Robinson et al. could be used at position 16 after the polarizer. A switcher of this type alternately rotates polarized light between two orthogonal polarization states, such as linear polarization states, to allow the presentation of two distinct images, one to each eye, while the user wears polarized glasses.
Real-D systems historically have utilized left and right circularly polarized light, where the glasses are made of a combination ¼ wave retarder plus a polarizer to change the circularly polarized light back to linearly polarized light before blocking one state. This apparently is less sensitive to head tilt and the achromatic polarization switcher is easier to fabricate. The glasses, however, add expense over embodiments that simply use a polarizer. In either case, the display screen must substantially maintain the polarization state of the incident image-bearing light and is, therefore, typically silvered. Silvered screens are more costly and exhibit angular sensitivity for gain. While this system is of some value, there is a significant light loss with MEMS based systems since they require polarization, which reduces the output in half. Similarly, there is additional light loss and added cost from the polarization switcher. LCOS based projectors are advantaged in that the output is typically already polarized in most configurations. These projectors are commonly more costly due to the difficulty of maintaining high polarization control through high angle optics. Therefore any gains in efficiency are offset by other costs.
A continuing problem with illumination efficiency relates to etendue or, similarly, to the Lagrange invariant. As is well known in the optical arts, etendue relates to the amount of light that can be handled by an optical system. Potentially, the larger the etendue, the brighter the image. Numerically, etendue is proportional to the product of two factors, namely the image area and the numerical aperture. In terms of the simplified optical system represented in FIG. 2 having light source 12, optics 18, and a spatial light modulator 20, etendue is a factor of the area of the light source A1 and its output angle θ1 and is equal to the area of the modulator A2 and its acceptance angle θ2. For increased brightness, it is desirable to provide as much light as possible from the area of light source 12. As a general principle, the optical design is advantaged when the etendue at the light source is most closely matched to the etendue at the modulator.
Increasing the numerical aperture, for example, increases etendue so that the optical system captures more light. Similarly, increasing the source image size, so that light originates over a larger area, increases etendue. In order to utilize an increased etendue on the illumination side, the etendue must be greater than or equal to that of the illumination source. Typically, however, the larger the image, the more costly. This is especially true of devices such as LCOS and DLP components, where the silicon substrate and defect potential increase with size. As a general rule, increased etendue results in a more complex and costly optical design.
Efficiency improves when the etendue of the light source is well-matched to the etendue of the spatial light modulator. Poorly matched etendue means that the optical system is either light-starved, unable to provide sufficient light to the spatial light modulators, or inefficient, effectively discarding a substantial portion of the light that is generated for modulation.
The goal of providing sufficient brightness for digital cinema applications at an acceptable system cost has eluded designers of both LCD and DLP systems. LCD-based systems have been compromised by the requirement for polarized light, reducing efficiency and increasing etendue, even where polarization recovery techniques are used. DLP device designs, not requiring polarized light, have proven to be somewhat more efficient, but still require expensive, short lived lamps and costly optical engines, making them too expensive to compete against conventional cinema projection equipment.
In order to compete with conventional high-end, film-based projection systems and provide what has been termed electronic or digital cinema, digital projectors must be capable of achieving comparable cinema brightness levels to this earlier equipment. As some idea of scale, the typical theatre requires on the order of 10,000 lumens projected onto screen sizes on the order of 40 feet in diagonal. The range of screens requires anywhere from 5,000 lumens to upwards of 40,000 lumens. In addition to this demanding brightness requirement, these projectors must also deliver high resolution (2048×1080 pixels) and provide around 2000:1 contrast and a wide color gamut.
Some digital cinema projector designs have proved to be capable of this level of performance. However, high equipment cost and operational costs have been obstacles. Projection apparatus that meet these requirements typically cost in excess of $50,000 each and utilize high wattage Xenon arc lamps that need replacement at intervals between 500-2000 hours, with typical replacement cost often exceeding $1000. The large etendue of the Xenon lamp has considerable impact on cost and complexity, since it necessitates relatively fast optics to collect and project light from these sources.
One drawback common to both DLP and LCOS LCD spatial light modulators (SLM) has been their limited ability to use solid-state light sources, particularly laser sources. Although they are advantaged over other types of light sources with regard to relative spectral purity and potentially high brightness levels, solid-state light sources require different approaches in order to use these advantages effectively. Conventional methods and devices for conditioning, redirecting, and combining light from color sources, used with earlier digital projector designs, can constrain how well laser array light sources are used.
Solid-state lasers promise improvements in etendue, longevity, and overall spectral and brightness stability but, until recently, have not been able to deliver visible light at sufficient levels and at costs acceptable for digital cinema. In a more recent development, VCSEL (Vertical Cavity Surface-Emitting Laser) laser arrays have been commercialized and show some promise as potential light sources. However, brightness is not yet high enough; the combined light from as many as nine individual arrays is needed in order to provide the necessary brightness for each color.
There are other difficulties with conventional approaches using solid-state arrays for digital projectors. A monolithic array of coherent lasers could be used, for example, such as the microlaser array described in U.S. Pat. No. 5,704,700 entitled “Laser Illuminated Image Projection System and Method of Using Same” to Kappel et al. With this type of approach, the number of lasers is selected to match the power requirements of the lumen output of the projector. In a high lumen projector, however, this approach presents a number of difficulties. Manufacturing yields drop as the number of devices increases and heat problems can be significant with larger scale arrays. Coherence can also create problems for monolithic designs. Coherence of the laser sources typically causes artifacts such as optical interference and speckle. It is, therefore, preferable to use an array of lasers where coherence, spatial and temporal coherence is weak or negligible. While spectral coherence is desirable from the standpoint of improved color gamut, a small amount of spectral broadening is also desirable for reducing sensitivity to interference and speckle and also lessens the effects of color shift of a single spectral source. This shift could occur, for example, in a three-color projection system that has separate red, green and blue laser sources. If all lasers in the single color arrays are connected together and of a narrow wavelength, and a shift occurs in the operating wavelength, the white point and color of the entire projector may fall out of specification. On the other hand, where the array is averaged with small variations in the wavelengths, the sensitivity to single color shifts in the overall output is greatly reduced. While components may be added to the system to help mitigate coherence, most means of reducing coherence beyond the source utilize components such as diffusers that increase the effective extent of the source (etendue). This can cause additional light loss and add expense to the system. Maintaining the small etendue of the lasers enables a simplification of the optical train for illumination, which is highly desirable.
Laser arrays of particular interest for projection applications are various types of VCSEL arrays, including VECSEL (Vertical Extended Cavity Surface-Emitting Laser) and NECSEL (Novalux Extended Cavity Surface-Emitting Laser) devices from Novalux, Sunnyvale, Calif. However, conventional solutions using these devices have been prone to a number of problems. One limitation relates to device yields. Due largely to heat and packaging problems for critical components, the commercialized VECSEL array is extended in length, but limited in height; typically, a VECSEL array has only two rows of emitting components. The use of more than two rows tends to dramatically increase yield difficulties. This practical limitation would make it difficult to provide a VECSEL illumination system for projection apparatus. In addition to these problems, conventional VECSEL designs are prone to difficulties with power connection and heat sinking. These lasers are of high power; for example, a single row laser device, frequency doubled into a two row device from Novalux produces over 3 W of usable light. Thus, there can be significant current requirements and heat load from the unused current. Lifetime and beam quality is highly dependent upon stable temperature maintenance.
Coupling of the laser sources to the projection system presents another difficulty that is not adequately addressed using conventional approaches. For example, using Novalux NESEL lasers, approximately nine 2 row by 24 laser arrays are required for each color in order to approximate the 10,000 lumen requirement of most theatres. It is desirable to separate these sources, as well as the electronic delivery and connection and the associated heat from the main thermally sensitive optical system to allow optimal performance of the projection engine. Other laser sources are possible, such as conventional edge emitting laser diodes. However, these are more difficult to package in array form and traditionally have a shorter lifetime at higher brightness levels.
Conventional solutions do not adequately address the problems of etendue-matching of the laser sources to the system and of thermally separating the illumination sources from the optical engine. Moreover, conventional solutions do not address ways to use polarized light from the laser devices more effectively.
Thus, it can be seen that there is a need for illumination solutions that capitalize on the advantages of polarized laser light sources for stereoscopic digital cinema projection systems.